1. Field of the Invention
The invention generally relates to a fuel and fuel reforming method, and in particular, to a fuel reformer for reforming hydrocarbon fuel to hydrogen-rich fuel and a fuel reforming method for reforming hydrocarbon fuel to hydrogen-rich fuel.
2. Description of the Prior Art
Conventionally, a type of reformer has been proposed which has a reforming section and a shift reaction section (for example, Japanese Patent Laid-Open Publication No. Hei 6-24702). In the reforming section, methanol is reformed to hydrogen-rich gas by reacting at a temperature between 300 and 400xc2x0 C., and in the shift reaction section, carbon monoxide in the hydrogen-rich gas, reformed at the reforming section, is reacted at a temperature between 200 and 300xc2x0 C. to increase the percentage of hydrogen present in the hydrogen-rich gas. When methanol is reformed to hydrogen-rich gas by the following equations 1 and 2, the reaction represented by equation 1 is an endothermic reaction and the reaction rate is fast, but the reaction represented by equation 2 is an exothermic reaction and the reaction rate is slow. Because of this, the shift reaction represented by equation 2 cannot sufficiently be performed by the reforming section alone, and therefore the reformer has two reaction layers with different temperatures.
CH30Hxe2x86x92CO+2H2 xe2x80x83xe2x80x83(Equation 1) 
CO+H20xe2x86x92C02+H2xe2x80x83xe2x80x83(Equation 2) 
However, in these conventional reformers, the shift reaction section is required to have a maximum capacity corresponding to the maximum capacity of the reforming section in order to sufficiently perform the shift reaction represented by equation 2, causing the size of the reformer to be large. This increase in size of the reformer causes problems in which a large quantity of energy and a long period of time are required to sufficiently increase the temperature of the shift reaction section at the time of start-up. In addition, the increase in size of the reformer causes another problem that the efficiency is reduced when the reformer is operating in a region much less than the maximum capacity.
The reformer and reforming method according to the present invention are directed to solving the problems caused by the increase in size of the reformer, and to providing a compact and efficient fuel reformer and a fuel reforming method for providing such a compact and efficient fuel reformer.
The fuel reformer according to the present invention implements the following in order to solve at least some of the objectives described above.
The fuel reformer of the present invention is a fuel reformer for reforming hydrocarbon fuels to hydrogen-rich fuel, comprising a reforming section for reforming the hydrocarbon fuel to hydrogen-rich gas, a shift reaction section, having a maximum capacity in a predetermined ratio to the maximum capacity of the reforming section, for reacting a portion of carbon monoxide present in the hydrogen-rich gas with water and shifting the carbon monoxide to hydrogen and carbon dioxide, an oxidation gas supplier for supplying an oxidation gas containing oxygen to the shift reaction section, and control means which determines excess, with respect to the maximum capacity of the shift reaction section, of the hydrogen-rich gas supplied from the reforming section to the shift reaction section and when the excess is determined, the control means controls the oxidation gas supplier to supply the oxidation gas to the shift reaction section using the oxidation gas supplier.
With the reformer according to the present invention, when the supplied amount of hydrogen-rich gas from the reforming section does not exceed the maximum capacity of the shift reaction section, the percentage of hydrogen present in the hydrogen-rich gas is increased using the shift reaction represented by equation 2 at the shift reaction section, but when the supplied amount of the hydrogen-rich gas to the shift reaction section exceeds the maximum capacity of the shift reaction section, the percentage of the carbon monoxide present in the hydrogen-rich gas is reduced by oxidizing a portion of the carbon monoxide present in the hydrogen-rich gas using the oxygen present in the supplied oxidation gas. Here, predetermined ratio is determined by operative characteristics of the devices (for example, fuel cell and hydrogen engine) which receive the hydrogen-rich gas supply from these fuel reformers. For example, this value can be the average value when the device receiving the hydrogen-rich gas supply is operated, or a value little higher or lower than the average value, a central value at the operating condition, or a value little higher or lower than the central value, or a value derived from overall efficiency of the device.
With the reformer according to the present invention, by setting the maximum capacity of the shift reaction section in a predetermined ratio to the maximum capacity of the reforming section, the shift reaction section can be made compact. As a result, the energy required to increase the temperature of the shift reaction section at start-up can be reduced and at the same time the required period of time can be shortened. Moreover, because the shift reaction section is compact, the reduction in efficiency when operating the fuel reformer at a region much smaller than the maximum capacity of the reforming section can be prevented, thereby improving the overall efficiency of the reformer.
In the fuel reformer according to the present invention, the control means can also control the oxidation gas supplier to set the molar ratio of the carbon monoxide present in the hydrogen-rich gas supplied to the shift reaction section to the oxygen present in the oxidation gas supplied to the shift reaction section to a predetermined molar ratio. In this manner, the amount of oxidation reaction with respect to the shift reaction can be restricted. In the fuel reformer according to this aspect, the molar ratio can be set to a molar ratio of more than 0.05 or more than 0.2. This molar ratio can be determined based on the carbon monoxide concentration permitted by the device which receives the hydrogen-rich fuel from the fuel reformer.
Moreover, the fuel reformer according to the present invention further comprises a gas flow amount detecting means for detecting flow amount of the hydrogen-rich gas supplied to the shift reaction section wherein the control means can decide the excess based on whether or not the flow amount of the hydrogen-rich gas detected by the gas flow amount detecting means exceeds the maximum capacity of the shift reaction section. In this manner, the oxidation gas supplied to the shift reaction section can be controlled based on the flow amount of hydrogen-rich gas supplied to the shift reaction section. In the fuel reformer according to this aspect, the gas flow amount detecting means can detect the flow amount of the hydrogen-rich gas supplied to the shift reaction section based on the supplied amount of the hydrocarbon fuel to the reforming section. In this manner, it can be determined if the flow amount of the hydrogen-rich gas exceeds the maximum capacity of the shift reaction: section from the amount of the hydrocarbon fuel supplied to the reforming section, and the oxidation gas supplied to the shift reaction section can be controlled based on the supplied amount of the hydrocarbon fuel.
Alternatively, the fuel reformer according to the present invention further comprises a carbon monoxide concentration sensor for detecting ct, the carbon monoxide concentration, in the hydrogen-rich gas after the reaction at the shift reaction section, wherein the control means determines whether the hydrogen-rich gas supplied to the shift reaction section exceeds the maximum capacity of the shift reaction section based on the carbon monoxide concentration detected by the carbon monoxide concentration sensor. In this manner, the carbon monoxide concentration in the hydrogen-rich gas after the reaction at the shift reaction section can be controlled with high precision. In this aspect of the fuel reformer, the control means can control the oxidation gas supplier to make the carbon monoxide concentration detected by the carbon monoxide concentration sensor less than a predetermined value. In this manner, the carbon monoxide concentration in the hydrogen-rich gas after the reaction at the shift reaction section can be reduced to a value less than a predetermined concentration.
Furthermore, in the fuel reformer of the present invention, if the capacity of the shift reaction section is represented in terms of the volume of the catalyst holding member holding the catalyst necessary for the shift reaction, the predetermined ratio can be a volume less than 50%. In this manner, the shift reaction section can be made small, about half size.
In the fuel reformer of the present invention, the catalyst for shift reaction at the shift reaction section can be copper-zinc catalyst.
The fuel reforming method according to the present invention is a fuel reforming method for reforming hydrocarbon fuel to hydrogen-rich fuel, comprising the steps of (a) reforming the hydrocarbon fuel to hydrogen-rich gas, (b) reacting a portion of carbon monoxide present in the reformed hydrogen-rich gas with water to shift to hydrogen and carbon dioxide, and (c) mixing oxidation gas containing oxygen with the hydrogen-rich gas and supplying the mixture to the reaction layer when supplied amount of hydrogen-rich gas reformed on the step (a) exceeds the capacity of the reaction layer used for step (b).
In this fuel reforming method according to the present invention, when supplied amount of the hydrogen-rich gas does not exceed the capacity of the reaction layer used for step (b), percentage of hydrogen present in the hydrogen-rich gas is increased by the shift reaction represented by equation 2 in step (b), and when supplied amount of the hydrogen-rich gas exceeds the capacity of the reaction layer used for step (b), oxidation gas is mixed with the hydrogen-rich gas, oxidizing a portion of carbon monoxide present in the hydrogen-rich gas, thereby reducing the percentage of carbon monoxide present in the hydrogen-rich gas. As a result, the reaction layer used for step (b) can be made compact, and the required energy and time to increase the temperature of the reaction layer used for step (b) can be reduced. Furthermore, because the reaction layer used for step (b) is compact, the reduction in the efficiency which occurs when fuel is reformed in a region much lower than the maximum capacity of the reaction layer used for step (a) can be prevented, and the overall efficiency of the reformer can be improved.
In the fuel reforming method according to the present invention, step (c) further comprises the steps of (C1) detecting flow amount of hydrogen-rich gas supplied to the reaction layer, (c2) determining whether or not the supplied amount of the hydrogen-rich gas exceeds the capacity of the reaction layer based on the detected flow amount of hydrogen-rich gas, and (c3) mixing the oxidation gas based on the flow amount of hydrogen-rich gas. Also, step (c) of the fuel reforming method according to the present invention further comprises the steps of (c1) detecting carbon monoxide concentration in the hydrogen-rich gas after the step (b), (c2) determining whether or not the supplied amount of the hydrogen-rich gas exceeds the capacity of the reaction layer based on the detected carbon monoxide concentration, and (c3) mixing the oxidation gas based on the carbon monoxide concentration.
In the fuel reforming method according to the present invention, the capacity of the reaction layer used for the step (b) can be made to be less than 50% of the maximum amount of the hydrogen-rich gas supplied from the step (a). Furthermore, in fuel reforming method according to the present invention, the shift reaction of step (b) can be performed using copper-zinc catalyst.